Space, weight and costs are critical considerations for forestry, lawn and garden equipment such as chain saws, line trimmers and blowers (generally referred to herein as portable power equipment) that are carried or handled by a consumer. Portable power equipment is thus typically powered with a two-stroke, single cylinder engine that is cooled by air blown across the cylinder. A muffler is directly fastened by bolts, springs or other retention devices to the exterior of the cylinder at an exhaust port defined through the wall of the cylinder. The engine is often enclosed to protect a user from the heat of the engine and to protect the engine from the oftentimes very dirty environment in which the equipment is being used. Air is blown through the housing by a flywheel fan. By placing the muffler on the downstream side of the cylinder, relatively more of the cylinder is exposed to the cooling air.
To reduce pollutant emissions from internal combustion engines, it is desirable to maintain the exhaust gases at relatively high temperatures. At high temperatures, unburnt fuel in the exhaust gases continues to combust after leaving the cylinder, thereby decreasing the amount of carbon monoxide and hydrocarbons emitted by the internal combustion engine.
Cylinders and exhaust systems that are much colder than the exhaust gases will cool the exhaust gases, resulting in less combustion in the exhaust gases and increased emission of pollutants. For example, during starting, the engine is cold. Thus, the exhaust gases initially lose much heat to the cylinder and are too cool for combustion. A liner fabricated from a quick-heating refractory material, such as that shown in U.S. Pat. No. 3,983,696 of Pflugfelder, reduces heat initially lost by the exhaust gases to the cylinder by rapidly heating and thereby reducing the temperature gradient between the exhaust gases and the material contacting the gases.
Similarly, in engines that are water-cooled, the coolant maintains the normal operating temperature of the cylinders at a relatively low level compared to the exhaust gases. The temperature of the exhaust gases consequently tends to be lowered to a point that is too cool for good combustion. A solution has been to insulate the exhaust gases from the cylinder by insulating the conduit from the exhaust port in the cylinder. Such solutions are shown, for example, in U.S. Pat. No. 4,031,699 of Suga et al. and U.S. Pat. No. 4,050,244 of Morikawa.
Generally, air cooled, two-stroke engines that power portable equipment do not have a problem with cold exhaust gases. The engine, and thus the exhaust gases, usually run hot, especially in warm weather when such equipment is used predominantly.